1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus.
2. Description of the Related Art
A motor-driven power steering apparatus may include a system which detects steering torque of a steering system by a steering torque sensor, and sets a target current on the basis of the detected steering torque. The system may control driving of an electric motor on the basis of a pulse width modulation (PWM) signal obtained by applying a proportional integral (PI) compensation to a deviation between the target current and a current actually flowing through the electric motor so as to execute a linear stroke of a rack shaft, and may apply a steering assist force to a tire wheel connected to the rack shaft, as described in Japanese Patent Application Laid-open No. 6-8839 (patent document 1).
In the conventional motor-driven power steering apparatus, even if slip is generated in a rotational force transmission system extending from the electric motor to the rack shaft, it is impossible to achieve early detection of the slip generation, and there is a risk that the steering assist control becomes unstable. With slip generated in the rotational force transmission system of the electric motor, there may be breakage of a belt transmission mechanism for converting the rotation of the electric motor into the linear stroke of the rack shaft and a ball screw apparatus, or an irregular slip caused by slack in a fastening connection portion of the transmission mechanism.